Gold Nanoparticles with N‐Heterocyclic Carbene/Triphenylamine Surface Ligands: Stable and Electrochromically Active Hybrid Materials for Optoelectronics

Abstract Organic‐hybrid particle‐based materials are increasingly important in (opto)electronics, sensing, and catalysis due to their printability and stretchability as well as their potential for unique synergistic functional effects. However, these functional properties are often limited due to poor electronic coupling between the organic shell and the nanoparticle. N‐heterocyclic carbenes (NHCs) belong to the most promising anchors to achieve electronic delocalization across the interface, as they form robust and highly conductive bonds with metals and offer a plethora of functionalization possibilities. Despite the outstanding potential of the conductive NHC‐metal bond, synthetic challenges have so far limited its application to the improvement of colloidal stabilities, disregarding the potential of the conductive anchor. Here, NHC anchors are used to modify redox‐active gold nanoparticles (AuNPs) with conjugated triphenylamines (TPA). The resulting AuNPs exhibit excellent thermal and redox stability benefiting from the robust NHC‐gold bond. As electrochromic materials, the hybrid materials show pronounced color changes from red to dark green, a highly stable cycling stability (1000 cycles), and a fast response speed (5.6 s/2.1 s). Furthermore, TPA‐NHC@AuNP exhibits an ionization potential of 5.3 eV and a distinct out‐of‐plane conductivity, making them a promising candidate for application as hole transport layers in optoelectronic devices.


Nuclear Magnetic Resonance (NMR) Spectroscopy
A Bruker Avance III 500 spectrometer was used to record 1 H NMR spectra at 500 MHz and 13 C NMR spectra at 126 MHz.The spectra were referenced to the residual solvent signals (CDCl 3 : δ( 1 H) = 7.26 ppm, DMSO-d 6 : δ( 1 H) = 2.50 ppm).The following abbreviations were used for 1 H NMR spectra data as listed: s -singlet, d -doublet, dd -doublet of doublet, t -triplet and m -multiplet.

Cyclic voltammetry (CV)
Electrochemical analyses were carried out on the Autolab PGSTAT302N workstation.Cyclic

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voltammetry (CV) measurements were carried out in a 3-electrode setup with platinum counter electrode, Ag/Ag+ reference electrode and platinum working electrodes.0.1 M NaCl/HCl was used as the electrolyte.PANI@ NPs coated on ITO substrates were used as working electrodes.

Transmission Electron Microscopy (TEM)
TEM images were obtained using a Zeiss Libra 120 with an accelerating voltage of 120 kV.Samples were prepared by placing a 2 µL droplet of the diluted NP dispersions or the assembly film on TEM grids (Cu, 200 Mesh, coated with carbon film; Science Services GmbH).

Scannning Electron Microscopy (SEM)
The surface morphology of NP films was measured using a Zeiss SEM NEON 40 EsB CrossBeam in the in-lens mode with an accelerating voltage of 3.0 kV.

Conductive Atomic Force Microscopes (C-AFM)
C-AFM measurements on spin-coated NP films were performed in PeakForce TUNA mode on a Dimension Icon AFM (Bruker, USA).We used a PeakForce TUNA cantilever with a spring constant of k = 0.4 N/m and a conductive Ti/Au coating.In this measurement mode, the tip is oscillating with a low frequency of 1 kHz and the current is detected during the time of contact between tip and sample using a current amplifier.All measurements were performed consecutively with the same cAFM tip starting from low tip bias (0.3 V) to higher tip bias (1 V).Due to capacitive instabilities in the current amplifier, we performed a baseline correction of the current channel for all measurements.Moreover, the system features a 7.5 mV voltage offset which was subtracted from all measurement biases.

Fabrication of electrochromic devices
ITO glass (thickness of ITO ≈ 110 nm; resistance, 20 Ω / square) was cleaned by washing with Milli-Q water, acetone and isopropanol, respectively.The gel electrolyte was prepared by mixing poly(methyl methacrylate) (PMMA) powder (0.2 g) and tetrabutylammonium tetrafluoroborate (TBATFB) (32.9 mg) in 1 mL propylene carbonate, and then heated at 50 °C until the solution became clear.Then Au NPs (3 mg) in 0.2 mL CHCl 3 was added to the above gel electrolyte.The solvent was removed by vacuum evaporation to form a homogeneous gel.After that, the gel electrolyte containing Au NP was spread on ITO with a double-sided tape frame as the spacer, and another ITO substrate was covered on top, followed by sealing via a UV-curing adhesive.The effective electrochromic area

Spectroelectrochemistry
Spectroelectrochemical properties were assessed by the combination of the electrochemical workstation (applying potentials) and the UV/vis/NIR spectrophotometer Cary 5000 (collecting the spectra with the range from 300 to 900 nm).

X-ray and Ultra-Violet Photoemission spectroscopy (XPS and UPS)
The samples were transferred to an ultrahigh vacuum chamber (ESCALAB 250Xi by Thermo Scientific, base pressure: 2 × 10 −10 mbar) for XPS measurements.XPS measurements were carried out using an XR6 monochromated Al K source (h = 1486.6eV).A pass energy of 50 eV and 20 eV was used for survey and core level spectra, respectively.Ultraviolet photoelectron spectroscopy (UPS) measurements were carried out using a double-differentially pumped He discharge lamp (hv = 21.22 eV) with a pass energy of 2 eV and a bias at −5 V.

Materials and Solution Preparation for Solar Cell Experiments
All used reagents and solvents were purchased from commercial suppliers and used without further purification, unless noted otherwise.Perovskite films and devices were fabricated using PbI 2 and PbBr 2 (99.99% purity) purchased from TCI, organic halide salts were purchased from GreatCell Solar Materials, and cesium iodide (99.99% purity) was purchased from Alfa Aesar.The PC 60 BM was purchased from Luminescence Technology Corporation (Lumtec).The bathocuproine (BCP; sublimed grade, 99.99% purity) was purchased from Sigma-Aldrich.All the anhydrous solvents were purchased from Acros Organics.
The TPA-NHC@Au NP (used as hole transporting layer; HTL) were dissolved in anhydrous chloroform at room temperature inside the nitrogen filled glovebox with different concentrations: 10 mg/mL, 7 mg/mL and 3 mg/mL.The perovskite precursor solution (1.2 M) contained mixed cations (Pb, Cs, FA, and MA) and halides (I and Br) dissolved in a solvent mixture (DMF/DMSO = 4/1) according to a formula of Cs 0.05 (FA 5/6 MA 1/6 ) 0.95 Pb(I 0.85 Br 0.15 ) 3 with an excess of PbI 2 of 1%.The PC 60 BM (used as electron transport layer; ETL) was dissolved in anhydrous chlorobenzene with a concentration of 20 mg/mL and kept for overnight stirring at 70 0 C inside the nitrogen filled glovebox.
The BCP solution was dissolved in anhydrous isopropanol with a concentration of 0.5 mg/mL and kept for overnight stirring at 70 0 C inside the nitrogen filled glovebox.

Device Fabrication and Characterization for Solar Cell Experiments:
Prepatterned ITO/glass substrates were sequentially cleaned with deionized water, acetone and 2propanol (IPA) by ultrasonication for 10 min in each solvent.The ITO/glass substrates were then dried with N 2 and treated with oxygen plasma at 100 mW for 10 min.The TPA-NHC@Au NP solution was spin-coated over cleaned ITO/glass substrate at 4000 rpm for 30 seconds and annealed at 70 0 C for 5 minutes on a hotplate inside a nitrogen filled glove box.The as-prepared HTL coated ITO substrates were transferred to drybox (relative humidity (RH) <1%) for perovskite deposition.The perovskite layer was deposited via a two-step spin-coating procedure with 1000 rpm for 10 s and 6000 rpm for 30 s. 150 μl of chlorobenzene was dripped on the spinning substrate during the last 5 seconds of the second spin-coating step.Subsequently, the samples were annealed at 100°C for 30 min.For the ETL deposition, the perovskite films are again transferred to the nitrogen filled glovebox.The PC 60 BM solution was spin-coated over the perovskite layer at 2000 rpm for 30 seconds (with a ramping speed of 1000 rpm/s) and kept for bench dry for 10 minutes.After that the BCP solution was spin-coated at 4000 rpm for 30 seconds (with a ramping speed of 1000 rpm/s) as hole-blocking layer.Finally, 80nm Ag was deposited under a vacuum of 4 X 10 -7 mbar.The device area was defined as 4.5 mm 2 by metal shadow mask.
Current density-voltage (J-V) measurements were performed in ambient conditions under simulated AM 1.5 light with an intensity of 100 mW cm −2 (Abet Sun 3000 Class AAA Solar Simulator).The intensity was calibrated using a Si reference cell (NIST traceable, VLSI).Devices were scanned using a Keithley 2450 source measure unit from -0.5 to 1.2 V and back, with a step size of 0.05 V and a dwell time of 0.1 s.The pixel area was 3 mm by 1.5 mm. a degassed toluene/ethanol/water mixture (8:1:1 v/v/v) was stirred at 100 °C for 24 h under nitrogen atmosphere.After cooling to room temperature, the mixture was extracted with dichloromethane and washed successively with water (3 x 20 mL) and a brine solution (20 mL).After drying with anhydrous magnesium sulfate, the organic layer was filtered and evaporated under reduced pressure.